Semiconductor devices communicate with their environment by accepting electrical impulses supplied by an external circuit (such as on a circuit board) and conducting these impulses to electrical circuits contained on a semiconductor chip. The semiconductor chip reacts to the input impulses in a predetermined manner to generate output electrical impulses. These output electrical impulses are then sent from the chip to an external circuit (e.g., on the circuit board). The input and output of electrical impulses to the semiconductor device occur over multiple paths of electrically conducting material, commonly referred to as leads.
The connections between the integrated circuit die and the leads can be made by wire bonding, in which a thin connecting wire is bonded at one end to the die input or output, and at the opposite end to the electrical lead. When wire bonding is used to connect an integrated circuit and its associated leads, a protective rigid housing is often added to enclose and protect the fragile connecting wires.
Alternatively, the connections between the integrated circuit die and the leads can be made by tape automated bonding (TAB) connections, in which a bead of conductive material connects the die input or output to the electrical lead. As shown in FIG. 1a, such semiconductor device packages 10 generally have a flexible tape substrate 12 that includes an insulative layer, and lead layer including a plurality of electrical leads. An integrated circuit chip 18 is attached approximately medially within the package. The view of FIG. 1b (taken through line 1b--1b of FIG. 1a) shows the semiconductor device package 10 in cross-section. The tape substrate 12 includes a patterned insulative layer 14, and a patterned conductive layer, or lead layer 16, including a plurality of electrical leads 17. An integrated circuit chip 18 which has at least one exposed surface 20 is connected to the lead layer 16 with a connective bead 22. An electrically nonconductive protective material 23, such as a standard semiconductor packaging encapsulant, can be present to maintain the integrated circuit chip 18, the lead layer 16, and the connective bead 22 in relative position. A rigid supporting member (not shown) can be present surrounding the periphery of the tape substrate 12.
A variety of manufacturing or handling-induced failures can cause failure of the electrical connection between the integrated circuit chip 18 and its associated electrical leads 17. An integrated circuit chip 18 can have a malformed or non-functional input/output site. A connecting TAB bead 22 can be absent or malformed, or can have internal impurities that reduce the conductivity of the connection between the integrated circuit chip 18 and the electrical lead 17. Ink, dust, or other surface contaminants present on the surface of the integrated circuit chip 18, the connective bead 22, or the electrical lead 17 can restrict or eliminate conductivity between the integrated circuit chip 18 and the electrical lead 17. The electrical leads 17 can be malformed, or can break due to mishandling. An abrupt shock can cause breakage or separation of the connective link between the integrated circuit chip 18, the connective bead 22, or the electrical lead 17, even if the lead connections had been previously tested and found to be adequate.
Prior art testing methods for packaged integrated circuit devices are time consuming, and most methods result in the destruction of the device tested. For example, lead connections can be tested after packaging by removal of any encapsulant, and conducting a pull test on the bonds. A non-destructive alternate method uses a Slam microscope to determine lead connection integrity. However, such microscopes require trained personnel, are difficult to interpret, and are prohibitively expensive for generalized screening.